As is known in the field of rechargeable batteries, overcharging a battery may result in a detrimental overvoltage condition. Additionally, in certain battery types, overdischarging the battery may result in a detrimental undervoltage condition. Circuits for protecting battery cells from potentially damaging overvoltage and/or undervoltage conditions are known. Such circuits measure the voltage across one or more cells and terminate the charging and/or discharging of the cells in response to the cell voltage exceeding a predetermined level or falling below a predetermined level, respectively.
One such circuit is described in U.S. Pat. No. 5,304,915 to Sanpei et al. The Sanpei et al. circuit utilizes a pair of N-channel Field Effect Transistors (FETs) connected in series, with interconnected drain electrodes, for protecting two series-connected cells. When an overvoltage condition is detected, one of the FETs turns off to prevent a charge current from flowing to the cells; whereas, when an undervoltage condition is detected, the other FET turns off to prevent a discharge current from flowing from the cells. When either FET is turned off, current flow through the cells in the opposite direction is permitted, thereby permitting the cells to be charged when an undervoltage condition is sensed and permitting the cells to be discharged when an overvoltage condition is sensed. More particularly, when an undervoltage condition is sensed, charge current is permitted to flow through the conducting FET and a parasitic diode of the non-conducting FET. Under normal operating conditions (i.e., the absence of an overvoltage or undervoltage condition) the charge or discharge current flows through both of the series-connected FETs.